Solid particle erosion of high-temperature components is a major issue in steam turbine engines. Nozzle blocks, control stage blades and intermediate pressure blades are particularly susceptible to solid particle erosion. Erosion changes the airfoil geometry and results in a loss of turbine efficiency. Erosion also creates sharp notches which may, under certain vibratory loads, lead to fatigue failures. Studies have been conducted to understand the mechanism of erosion and to find ways of minimizing it. These include bypassing steam during start-up, altering the airfoil profiles and using erosion resistant coatings.
The most commonly used types of erosion coatings are boride and carbide. Boride coatings may be applied by diffusion. A component is embedded in a boron-containing material, held at an elevated temperature for sufficient time, cooled continuously to room temperature, and finally tempered at a temperature and time appropriate to the substrate alloy. Extensive research conducted on the subject suggests that it is virtually impossible to produce crack-free boride coatings for parts. Coating cracks significantly reduce the fatigue strength of the coated parts.
FIG. 1 is a continuous cooling transformation (CCT) diagram. Unlike isothermal transformation curves, which depend only upon fixed temperatures, CCT diagrams are concerned with both transformation time and temperature under certain cooling rates. Accordingly, CCT diagrams are useful for commercial heat treatments and in welding industries. In the prior art example of FIG. 1, the curves starting at a bonding temperature BT (i.e. a boriding or carbiding temperature), and sloping downward to the right, are sample cooling rates. The fastest cooling rate is shown by curve 22, and the slowest rate is shown by curve 24. Metallographic phases at various temperature ranges and cooling rates are marked on the diagram, and are identified in the legend. Curve 28 is a ferrite transformation range or C-curve, within which a substantial amount of ferrite transformation will occur, depending on the cooling rate. A slow-cooling curve 30 passes through the ferrite transformation range 28. A faster-cooling rate 26 passes the ferrite transformation curve 28 before any or any substantial amount of ferrite transformation can occur.
Many high-temperature steam turbine blades are made of 12% Cr type steels such as AISI 403, 422 and others. These alloys attain strength through martensitic transformation achieved by rapid cooling from the austenitizing temperature. The slowest cooling rate cannot be less than that required to avoid passing through the ferrite transformation curve. For example, X22CrMoV12.1 steel should be cooled from 1050 to 650 degrees C. in less than two hours, requiring a cooling rate greater than 200 degrees C. per hour. However, this minimum cooling rate required to attain strength is not slow enough to prevent the boride coating from developing cracks as illustrated in FIG. 2. Similarly, minimum cooling rate required to attain strength in AISI 422 is 400 degrees C. per hour.